Circulators are notably used in telecommunications or radar systems, according to the principle illustrated in FIG. 2. A telecommunications system mainly comprises a central signal processing part notably providing an attenuation function AT and a phase-shifting function D, typically implemented by digital electronic circuits (microchips), associated with a transmitter stage E, a receiver stage R and an antenna A.
The transmitter stage E mainly comprises an amplifier DRA (for “Digital Research Amplifier”), an amplifier HPA (for “High Power Amplifier”), and an isolator I. An isolator is a particular case of a circulator. A 50 ohms load is connected to one of the ports (often the port 3 by convention). Whatever the impedance of the circuit connected at the output on the second port p2, there is practically no return toward the transmitter (port p1): the major part of the returned or coupled power is dissipated by the load connected to p3. An isolator is generally used in order to limit as much as possible signal returns onto the output of the HPA. The reason for this is that any signal arriving on the output of the HPA could lead to a serious malfunctioning or even the destruction of this component.
The receiver stage R comprises a bandwidth limiter circuit LIM and a signal amplifier generally denoted LNA (Low Noise Amplifier).
A circulator C with three channels (or ports) p1, p2, p3 controlled by an electronic activation circuit, not shown, allows a radiofrequency signal supplied by the transmitter stage to be transferred to the antenna A (transmission p1 toward p2, p3 being isolated), or a signal picked up by the antenna to be transmitted to the receiver stage (transmission p2 toward p3, p1 being isolated).
The radiofrequency circulator C must notably meet the following constraints in its characteristics: have fast switching times; withstand the high radiofrequency power of the signals to be transmitted to the antenna; have limited insertion losses.
According to the prior art, the radiofrequency circulators used are bulky structures using a ferrite and a permanent magnet that impose a direction of electromagnetic gyration.
However, these ferromagnetic circulators have various drawbacks. They are very costly components. They are not easily reproducible, since they require human intervention for correct adjustment. Their structure is very bulky. They occupy around 80% of the space within a telecommunications system. They consume a large amount of electrical power, and consequently pose problems of thermal dissipation. They introduce insertion losses (radiofrequency power losses in the coupling across the ferrite) of the order of 2 to 4 dB within their operating frequency band, which furthermore is narrow, of the order of 0.2 to 1 GigaHertz.
For all these various reasons, it is desirable to replace these ferromagnetic circulators by components which do not exhibit these various drawbacks.